The present invention relates generally to combustion turbine rotor blades and more particularly to a cooled combustion turbine rotor blade which may be backfitted into a rotor disc originally structured for a non-cooled turbine rotor blade.
It is well established that greater operating efficiency and power output of a combustion turbine may be achieved through higher inlet operating temperatures. Inlet operating temperatures are limited, however, by the maximum temperature tolerable to the rotating turbine blades. Also, as turbine blade temperature increases with increasing inlet gas temperature, the vulnerability of the blades to damage from the tension and stresses which normally accompany blade rotation also increases. Cooling the turbine blades, or forming the blades from a temperature resistant material, or both, permits an increase in inlet operating temperatures while keeping the turbine blade temperature below the maximum specified operating temperature for the blade material.
There are presently many combustion turbines in the field today which have non-cooled turbine rotor blades. In some models the first stage blades are cooled while blades in subsequent stages are not. Generally, those combustion turbines which have non-cooled turbine rotor blades provide some means for cooling the root of the blades, such as the means set forth in U.S. Pat. No. 3,501,249 and U.S. Pat. No. 3,572,966. Cooling the blade root is a simple method for providing partial cooling of the blade airfoil. The latter patent describes a structure whereby cooling air drawn from the compressor is forced through individual channels in each disc to a path between the blade root and the disc. After passing between the blade root and the disc, the cooling air exits into the exhaust path of the hot motive gases driving the turbine.
To improve the operating efficiency and power output of the combustion turbine, it is desirable to provide means which enables a flow of cooling air through the turbine blades themselves so that the blade surfaces are positively cooled to keep the blade surface temperature below the turbine inlet temperature. A prior art approach to this problem is shown in U.S. Pat. No. 3,853,425. This patent describes an assembly for sealing the exhaust end of the cooling path between the blade root and the disc, forcing the cooling air up through the blade root into the airfoil portion of the turbine blade. The cooling air thereafter exits from the airfoil portion into the exhaust path of the hot motive gases. The sealing assembly of the latter patent comprises a seal structure mating with grooves within the blade root and the disc to close the exhaust end of the cooling path between the blade root and the disc. Hence, use of the apparatus described in the latter patent requires special machining of the rotor disc as well as a specially structured turbine blade.
While the sealing assembly described above provides an efficient and effective method for channeling cooling air to the turbine blade, it is not readily adaptable to combustion turbines presently in the field. Application of this structure to field units would require structural modifications to both the turbine blade root and the rotor disc. Modifications to the rotor disc would necessitate removal of the rotor spindle from the lower half of the turbine casing. As explained in U.S. Pat. No. 3,493,212, the positioning of the rotor is highly critical, so that once it is properly located it should remain undisturbed if at all possible. Removal of the rotor spindle also increases the time and expense required to implement changes to the combustion turbine.
Hence, it would be advantageous to develop a cooled combustion turbine blade with sealing structure such that it may be backfitted into presently existing rotor discs, thereby providing the advantages of cooled turbine blades without the disadvantages of rotor spindle removal.